(1) Field of the Invention
The present invention relates to a low-pressure turbine rotor, and to a steam turbine rotor with a large power generation capacity suitable for a large thermal power generation turbine or the like.
(2) Description of the Related Art
In recent years, enhancement of the efficiency of thermal power generation plants has been desired from the viewpoint of energy saving (for example, saving of fossil fuel) and prevention of global warming (for example, reduction of the generation amount of CO2 gas). One of effective means for enhancing the efficiency of steam turbines is to increase the length of last stage long blades of the steam turbines. Further, by increasing the length of the last stage long blades of the steam turbines, reduction of facility construction time period and cost reduction thereby can be also expected as secondary effects because of reduction of the number of turbine casings.
Because the long blades are used under high centrifugal stress and humid environments, the material of the long blades is required to have characteristics excellent in both strength and corrosion resistance. While rotors on which the blades are planted are also required to have high strength due to increase in size of the blades, low-pressure rotors (ASTM designation A470Class7) which are used at present are insufficient in strength. If mono-block type rotors are strengthened by heat treatment, the characteristics balance as rotors becomes worsened because the mono-block type rotors are needlessly strengthened and thus reduced in toughness except in last stages, and the sensitivity of stress-corrosion-cracking is enhanced.